Thunder is the sound caused by lightning. Depending on the distance and nature of the
lightning, thunder can range from a sharp, loud crack to a long, low rumble. The sudden
increase in pressure and temperature from lightning produces rapid expansion of the
air surrounding and within a bolt of lightning. In turn, this expansion of air creates a sonic
shock wave, similar to a sonic boom, which produces the sound of thunder, often referred
to as a clap, crack, or peal of thunder. The distance of the lightning can be calculated
by the listener based on the time interval from when the lightning is seen to when the
sound is heard.
Cause
The cause of thunder has been the subject of centuries of speculation and scientific
inquiry. The first recorded theory is attributed to the Greek philosopher Aristotle in the
third century BC, and an early speculation was that it was caused by the collision of
clouds. Subsequently, numerous other theories were proposed. By the mid-19th century, the
accepted theory was that lightning produced a vacuum.
In the 20th century a consensus evolved that thunder must begin with a shock wave in the
air due to the sudden thermal expansion of the plasma in the lightning channel. The temperature
inside the lightning channel, measured by spectral analysis, varies during its 50 μs
existence, rising sharply from an initial temperature of about 20,000 K to about 30,000 K,
then dropping away gradually to about 10,000 K. The average is about 20,400 K. This heating
causes it to expand outward, plowing into the surrounding cooler air at a speed faster
than sound would travel in that cooler air. The outward-moving pulse that results is a
shock wave, similar in principle to the shock wave formed by an explosion, or at the front
of a supersonic aircraft. Experimental studies of simulated lightning
have produced results largely consistent with this model, though there is continued debate
about the precise physical mechanisms of the process. Other causes have also been proposed,
relying on electrodynamic effects of the massive current acting on the plasma in the bolt of
lightning. The shockwave in thunder is sufficient to cause injury, such as internal contusion,
to individuals nearby. Inversion thunder results when lightning strikes
between cloud and ground occur during a temperature inversion. In such an inversion, the air near
the ground is cooler than the higher air. The sound energy is prevented from dispersing
vertically as it would in a non-inversion and is thus concentrated in the near-ground
layer. Inversions often occur when warm moist air passes above a cold front; the resulting
thunder sound is significantly louder than it would be if heard at the same distance
in a non inversion condition.
Etymology The d in Modern English thunder is epenthetic,
and is now found as well in Modern Dutch donder. In Latin the term was tonare "to thunder".
The name of the Germanic god Thor comes from the Old Norse word for thunder.
The shared Proto-Indo-European root is *tón-r̥ or *tar-, also found Gaulish Taranis and Hittite
Tarhunt. Calculating distance
A flash of lightning, followed after some time by a rumble of thunder illustrates the
fact that sound travels significantly slower than light. Using this difference, one can
estimate how far away the bolt of lightning is by timing the interval between seeing the
flash and hearing thunder. The speed of sound in dry air is approximately 343 m/s or 1,127
ft/s or 768 mph at 20 °C. This translates to 0.213 miles per second, or approximately
5 seconds per mile. However, this figure is only an approximation, as air is hardly dry
in a thunderstorm. The speed of light is high enough that it
can be taken as infinite in this calculation because of the relatively small distance involved.
Therefore, the lightning is approximately one kilometer distant for every 3 seconds
that elapse between the visible flash and the first sound of thunder. In the same five
seconds, the light could have traveled the Lunar distance four times. Thunder is seldom
heard at distances over 20 kilometers. A very bright flash of lightning and an almost simultaneous
sharp "crack" of thunder, a thundercrack, therefore indicates that the lightning strike
was very near.
See also Thunderstorm
Thunderbolt Lightning
Brontophobia Mistpouffers
The Castle Thunder sound effect List of thunder gods
Thursday References
External links Media related to Thunder at Wikimedia Commons
The science of thunder Thunder: A Child of Lightning
Wikibooks: Engineering Acoustics/Thunder acoustics